A fountain pen is a writing instrument that uses liquid ink stored in a refillable or replaceable reservoir. The ink is generally fed from the reservoir to the pen's nib through a combination of capillary action and gravity.
As is well known, a large number of fountains pens use disposable ink cartridges as their ink source. An ink cartridge is typically sealed by a small ball that blocks the cartridge's ink flow hole until the cartridge is firmly inserted into a reservoir chamber within the pen, whereby a small pin pushes in the plastic ball and enables the ink to flow into the pen's ink feed system. When the ink from a cartridge has been exhausted, it is simply removed and replaced by a new one. Ink cartridges are popular because of their convenience, and many fountain pen manufacturers have ensured that standardized cylindrical-shaped ink cartridges in one (or both) of two sizes—a short cartridge approximately 38 mm in length and 0.75 ml in capacity and a long cartridge approximately 72 mm in length and 1.45 ml in capacity—are compatible with their pens.
In many cases, a converter may also be used in place of an ink cartridge. Converters generally are of the same size as an ink cartridge but also have a filling mechanism (typically screw or piston-based) attached to their reservoirs, thereby allowing them to be refilled from a bottle of ink. The use of converters is therefore normally more economical and environmentally friendly, and it may also enable a greater selection of inks to be used.
In many fountain pens, the ink reservoir chamber within the barrel is accessed by unscrewing, or otherwise separating, the nib from the barrel. In such cases, the insertion and extraction of cartridges and converters from the pen's barrel is often awkward for a user and may additionally leave ink marks on the user's hands. Furthermore, there is a heightened risk that the nib and/or feeding system of the pen may be damaged due to repeated handling by the user.
Thus, despite the convenience associated with using ink cartridges and converters (hereinafter referred to as ink reservoir units), there remains a need for a safer, more effective, and more user-friendly means of inserting and removing them in such fountain pens.